1. Technical Field of the Invention
The present invention relates generally to electronic apparatus including, but not limited to, electronic ignition devices (igniters) for use in internal combustion engines as mounted within engine room spaces of land vehicles, and more particularly to electronic devices having resin-sealed package structures.
2. Prior Art
One prior known structure of a semiconductor package for use in internal combustion engines, in particular, igniter modules, has been disclosed in Japanese Patent No. 2590601.
The prior art igniter package structure as taught thereby is designed so that a printed circuit board and a power transistor are integrally packed by transfer mold techniques using synthetic resin together with a heat release plate, known as a heat sink. In this prior art the power transistor packed is coupled to the heat sink via a heat spreader and dielectric plate lying therebetween.
Another prior art igniter device adapted for internal combustion engines is found in Japanese application patent laid-open publication No. Hei 8-69926, which is designed so that a controller circuit board and power switching device are built within the interior space of a heat sink as machined into a box-like shape. The power switching device is coupled thereto by a soldering material made of an Sn--Sb alloy on the heat sink, which alloy comprises a blend of tin (Sn) and antimony (Sb) at appropriate mixture ratios. The controller circuit board is bonded by silicon-based adhesive to the heat sink.
After having mounted these parts or components in the package structure, a silicon-based gel is injected into inside of the heat sink. The same Japanese document also discloses therein that the above-noted power transistor contact/junction technique using Sn--Sb alloy-based solder is also applicable to resin-sealed transistor package units (transfer-mold using epoxy) each including a power transistor only.
Still another prior art approach is disclosed in Japanese application patent laid-open publication No. Hei 9-177647, which teaches an ignition device having a semiconductor IC chip with function circuitry being integrated thereon.
Typically, electronic devices adaptable for use in internal combustion engines, such as igniters, are used under severe temperature conditions and, for the very reason, are the components that are strictly required to offer increased durability or robustness (long life-time). In addition, depending upon specifications required, some igniters are required to accommodate multiple functionalities.
These requirements are not met by the prior art one-chip IC package module discussed above. A need therefore exists to manufacture the power semiconductor device and its associated substrate or circuit board individually at separate process steps in order to fully satisfy the specification required.
In the semiconductor package structure for internal combustion engines, it is the power semiconductor switching device of significant self heat generation that is especially required to achieve the long lifetime issue.
More specifically, in view of the fact that currently available power semiconductor devices are typically made of silicon (Si), in cases where this Si is stacked on or over the heat sink, possible differences in linear expansion coefficient between Si and heat sink materials (for example, the linear expansion coefficient of Si is approximately 3.times.10.sup.-6 /.degree. C. whereas a heat sink made of Cu measures about 17.times.10.sup.-6 /.degree. C. in linear expansion coefficient) act to create unwanted stresses at soldered portions and thus are controlling the lifetime thereof.
With high-temperature Pb--Sn alloy-based solder materials (Pb:Sn=90:10) that have traditionally been employed as solders for use with electronic components of this type, these are not sufficient in absorbing such linear expansion coefficient differences and are thus remain difficult to attain long lifetime required. One known approach to avoiding this problem is to additionally employ a thermal relaxation member (made for example of molybdenum) which is laminated to reduce such linear expansion coefficient differences thereby making moderate or "relaxing" resultant stress forces.
It may be evaluated that in the event that the above-noted Pb--Sn alloy-based solder is replaced with an Sn--Sb alloy-based solder (for instance, made of Sn containing therein Sb at 5 to 8%, Ni at 0.0 to 0.8%, and P at 0.0-0.1%) for coupling or adhering between a power semiconductor device and its associated heat sink in a way as suggested by the above-identified Japanese application patent laid-open publication No. Hei. 8-69926.
This Sn--Sb alloy-based solder is inherently high in physical robustness or stiffness so that it becomes possible to decrease the propagation speed of thermal shocks occurring due to linear expansion coefficient differences between the power semiconductor device and the heat sink, which in turn makes it expectable to improve the durability at soldered portions.